This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. E. coli histidine kinase receptors (HKR) are proteins located in the cell membrane and their role is to sense environmental stimuli. In order to transmit the signal to the cell and trigger a response to the stimulus they work together with their intracellular partner proteins, known as response regulators (RR). HKRs and RRs form the so-called Two Component Systems (TCS), which are the predominant signal transduction mechanisms in prokaryotic organisms, also present in fungi and plants. Most HKRs have two transmembrane domains ,a periplasmic sensor domain, and a large cytoplasmic domain which is responsible for the transduction of the signal to the RR. There are 23 identified HKRs in E. coli but a structure of no full length protein is known.